The identification of genes necessary for establishing pattern formation during morphogenesis and the study of their regulation are problems which are central to many aspects of vertebrate biology. Many key processes in morphogenesis, including responses to trophic stimuli, cell-cell interactions, migration, differential cell multiplication, programmed cell death, etc., are also recapitulated in a pathologic manner during oncogenesis. Chick limb development is an attractive system for studying the molecular basis of pattern formation because critical events at the level of tissue/cellular interactions involved in pattern formation have been well characterized and appear to be very similar to those in mammalian systems, and this system is readily amenable to biochemical and molecular analysis as well as microsurgical manipulation. It is the aim of this long-term project to isolate genes that regulate morphogenesis in the chick embryo limb bud. Two general approaches are being developed: 1) the generation of subtracted cDNA libraries enriched for potential regulatory and induced genes; and 2) the identification of related/new members of conserved gene families that have been implicated in developmental regulatory processes in other systems. Currently, two new members of the homeobox gene family have been identified which are expressed predominantly in limb buds during early development. These genes each show spatially restricted expression domains within the limb bud that suggest roles in regulating pattern formation along the anterior-position (A-P) and proximodistal (P-D) axes of the limb. One of these homeobox genes also displays expression differences between wing and leg buds, indicating it may also regulate the determination of limb-type identity. Studies are underway to elucidate the function of these regulatory genes, using both molecular genetic and biochemical approaches.